1. Field of the Invention
This invention relates to a method and apparatus for screen printing for producing security foils for objects of value such as papers of value consisting of a transparent or translucent carrier foil and a cover layer applied thereto which is present in the form of characters or patterns and/or has coating-free areas in the form of characters, patterns or the like which are clearly recognizable in particular in transmitted light.
2. Related Art
Security foils in the form of threads or bands which are embedded in papers of value such as bank notes, checks, identity cards, credit cards and the like are known. To increase security and as protection from forgery such threads are frequently provided with so-called negative writing. Said writing is formed by metal-free areas in an otherwise all-over metallic coating of the transparent carrier material forming the threads. If one holds a paper of value containing such a thread against the light, the thread basically appears dark because of the opaque metallic coating. The metal-free areas stand out against this dark background as light and contrasting. This conspicuous incident/transmitted light effect provides very good protection from forgery. The protective effect of the negative writing lies in the elaborate and complicated production requiring great know-how, high equipment expense, special printing inks and a great number of operations. In contrast, usual printed information can be forged true to the original without great effort using copiers and printers available today.
Security foils are also known which have diffraction structures such as conventional holograms, cinegrams, pixelgrams, etc. The protection from forgery by said elements is based on the viewing angle-dependent interplay of colors which said elements show. Such security foils are frequently applied to the surface of objects of value in the form of strips or labels.
Mass production of the diffraction structures is usually effected by converting the diffraction structures into a relief structure which is subsequently used as an embossing die. Embossing is usually done in a thermplastic layer or a lacquer or resin layer curable during the embossing process. Said layers can possibly be provided before the embossing process with a thin metal layer which on the finished product ensures that the diffraction structures are recognizable in reflected light. However, the metalization process is frequently effected only after embossing, in particular when embossing is done in liquid to viscous materials such as UV-curable lacquers which are only cured during the embossing process by corresponding treatment, e.g. irradiation with UV light.
Said production of such embossed lacquer layers can be effected directly on the object to be protected. According to an alternative, very common technology, transfer foils containing the embossed lacquer layer in their layer structure are produced in a separate operation. This layer structure is subsequently transferred to the object of value by means of suitable adhesive layers.
To increase the protection from forgery it has also been proposed to provide the reflecting layer of such a diffraction structure with negative writing, i.e. with gaps in the form of characters, patterns or the like (EP 0 330 733 A1). EP 0 330 733 A1 also states different methods for producing such gaps. Besides conventional etching methods, reference is also made to a production method by which an ink having thermoplastic properties and softening and becoming gluey at higher temperatures is printed on the metal-coated side of the foil. If a thus pretreated foil is laminated against a second untreated sheet of foil using heat and pressure, the two foils stick together in the area of the printed characters or patterns. When the two foils are separated the areas corresponding to the characters or patterns are then detached from the metal coating.
For producing packaging material it is known from DE-OS 36 10 379 to first print a printed image on the foils as is to appear later as a negative image in the metal coating, and only apply the metal coating in a second method step. The printing method can be intaglio or flexographic printing. For applying the printed image one uses inks or lacquers having low adhesion to the metal coating. The metal coating is then removed mechanically by the action of an air or liquid jet or by a mechanical squeegee.
All said known methods have in common that the information which is later to be present in the form of gaps must be printed with poorly adhesive or etching inks in a certain operation. Separate printing forms must be produced for this printing operation. Production of the forms is very elaborate. For an intaglio cylinder for example, a metallic cylinder must be prepared very elaborately, i.e. copperplated, polished, provided with the information to be printed and finally chromium-plated. The printing cylinder can in addition only be used for the particular printing job. In particular for mass articles such as holograms, negative characters can therefore only be provided if an accordingly high number of holograms is produced so that the additional costs are distributed over as many final products as possible. For small runs or productions which are to be provided with different negative characters, the production costs are hitherto prohibitive.